Semiconductor substrate bonding typically involves contacting a first substrate with a second substrate and bonding the two substrates together via molecular adhesion. This type of bonding is common for Semiconductor On Insulator, (SeOI) type structures, and Silicon On Insulator (SOI) type structures. In this context, at least one of the substrates to be bonded presents a surface oxide layer; typically Si/SiO2 bonding or SiO2/SiO2 bonding is performed in order to form an SOI structure.
Molecular adhesion bonding allows two substrates with perfectly flat (“polished mirror”) surfaces to adhere to each other without applying adhesive (e.g., paste, glue). The substrate surfaces are generally those of substrates in electrically insulating materials (e.g., crystal, glass) or in semiconductor materials (e.g., Si, GaAs, SiC, Ge). Bonding is typically initiated by the local application of slight pressure on one or both of the substrates put in contact. A bonding wave then propagates over the entire span of the substrates in several seconds to bring the two substrates together at the atomic scale. The bonding energy obtained at room temperature with molecular adhesion bonding is generally rather low relative to that observed between two solids connected in a covalent, ionic, or metallic manner. Thus, to obtain satisfactory bonding of the two substrates to each other, usually one of the substrate surfaces to be bonded is prepared before bonding. This is to increase the mechanical strength and/or increase the bonding interface quality.
Preparation for bonding typically involves a chemical treatment, or cleaning, of the substrate surfaces. Cleaning provides removal of particles, removal hydrocarbons, removal of metal contaminants, a low surface roughness (typically less than 5 Å RMS) and high hydrophily—a high density of silanol bonds (Si—OH bonds) finishing the surfaces to be bonded. Cleaning before bonding can include RCA type cleaning, which is a combination of a Standard Clean 1 (SC1) bath including ammonium hydroxide (NH4OH), hydrogen peroxide (H2O2) and water (H2O), and is suitable for removing particles and hydrocarbons. Another example is a Standard Clean 2 (SC2) bath of hydrochloric acid (HCl), hydrogen peroxide (H2O2) and water (H2O), which is suitable for removing metal contaminants. Further examples are cleaning with an ozone (O3) solution suitable for removing organic contaminants, and cleaning with a Sulfuric Peroxide Mixture (SPM), a solution containing a mixture of sulfuric acid and oxygenated water. Monitoring the different cleaning parameters, particularly bath temperatures prevents certain defects from appearing at the bonding interface of the bonded structure.
SMART-CUT® methods are often used for transfer of a thin layer from the donor substrate to a receiver substrate. (See “Silicon Wafer bonding technology for VLSI and MEMS applications”, S. S. Iyer and A. J. Auberton-Hervé, IEE, 2002.) However, SMART-CUT® and similar methods are susceptible to the problem of edge voids, or holes in the thin layer that is transferred, located in the peripheral region of the receiver substrate. Edge voids are zones where transfer has not been successful, typically having a diameter between 50 μm and 2 mm located between 0.5 to 5 mm from the edge of the structure.
Edge voids are macroscopic defects due to poor edge bonding of the substrates. They are serious defects and generally are prohibitive. In fact, in the absence of a thin layer used as an active layer for forming electronic components at the location of an edge void, no component may be built at this location. Given the size of edge voids, an electronic component having at least one edge void is necessarily defective. FIG. 1 schematically shows a sectional view of an SOI structure including a receiver substrate 10, SiO2 layer 16, and top layer 18. Layers transferred on the receiver substrate 10 present a hole, or edge void 12, with a diameter typically between 50 μm and 2 mm, situated 1-5 mm from the edge of the structure.
Blister type defects can also occur during transfer. Blisters are circular macroscopic defects with a diameter typically between 0.1 mm and 3 mm, visible after transfer of a thin layer.
In the case of an SOI structure obtained by the SMART-CUT® method, bonding a receiver substrate (Si) on an oxidized and implanted donor substrate may lead to the formation of blisters on the structure obtained after the transfer. These blisters result from local separation between the receiver silicon and the oxide of the thin layer transferred, as shown in FIG. 2. Separation can result from possible particles, traces of hydrocarbons, or even surface irregularities (micro-roughness that is higher locally) at the surface of one or both of the substrates put in contact.
In the case where bonding is performed in view of a SMART-CUT® or similar layer transfer method (hereinafter “formation of an SOI”), blisters 14 created at the bonding interface between the donor substrate and the receiver substrate may expand during detachment annealing, damaging the useful layer of the final structure obtained after the transfer. The SOI structure of FIG. 2 includes receiver substrate 10, SiO2 layer 16, and top layer 18. As diagrammed in FIG. 2, blisters 14 may be located at the center or at the periphery of the structure. Blisters are prohibitive defects for an SOI.
Blister and edge void defects are connected to the bonding and preparation of the surface; cleaning prior to bonding can reduce or prevent the occurrence of these defects. For example, blister defects at the bonding interface can be prevented by using a low concentration SC1 bath, especially at low temperature of less than about 65° C. Edge void defects at the bonding interface after transfer can be prevented by using an SC1 bath at high temperature, typically greater than or equal to 70° C.
Unfortunately, the disparate temperatures required to address each defect do not allow for prevention of both the blister and edge void defects. Accordingly, there is a need in the art for a cleaning step that reconciles these two treatments and provides a bonding interface free of both blister and edge voids defects.